This proposal seeks funding to create and distribute a nationally representative 1 in 500 Public Use Microdata Sample of the 1880 United States Population Census. Public Use samples already exist for the census years 1900, 1910, 1940, 1950, 1960, 1970, and 1980. Although the historical samples for the period 1900-1950 have only been available for a few years, they have already led to an outpouring of new research on the nature of long-term social change. An 1880 census file will not only constitute an invaluable resource in its own right, but will also enhance the value of the previously created public use samples; used in combination, the eight data sets spanning a century of cataclysmic social and economic change will comprise our most important resource for the study of changing social structure. Microdata files allow researchers to make tabulations tailored to their specific research questions. Without individual-level data, even some of the basic questions about changing social structure may be unanswerable because of the incompatibility of published data for different census years. In addition, the public use samples have allowed researchers to move beyond simple tabular analysis and apply increasingly sophisticated multivariate techniques. The existence of these data have dramatically increased the power of quantitative social science research. By extending the series backwards, the 1880 public use sample will both provide a baseline for understanding the social change of the twentieth century and greatly improve our understanding of American society in the second half of the nineteenth century. The late nineteenth century is a critical period in the study of fertility decline, urbanization, immigration, morbidity, household composition, and occupational structure. The case for a sample from 1880 is especially compelling because although the original enumeration was of unprecedented quality, many of the most important variables were never tabulated. The enumerator's manuscripts therefore include a great deal of information on demography and social structure that can only be taken advantage of through the creation of a new microdata set. In addition to converting a sample of the 1880 population into machine- readable form, the project will evaluate the sample quality through consistency checks, random spot verification, and comparison with aggregate statistics in the published census volumes; edit and allocate missing, illegible, and inconsistent data through logical rules and imputation procedures; construct new variables on household composition and relationships within families; create alternative coding schemes to ensure that the 1880 sample is compatible with all other public use samples currently available; and prepare documentation for the user file, including detailed descriptions of the sampling and data processing procedures, and a guide to the use of the sample.